Wye-Wye Transformer

[ron]

Specifications required a Delta-Wye Grounded transformer to serve a distribution panel with both three phase (208V) and single phase (120V) loads.
Contractor installed Wye-Wye grounded transformer (leftover at the distributor), where the secondary Xo is grounded, but the Xo of the Primary is not grounded. In addition, a neutral was not pulled with the primary conductors.

Other than the obvious of being used outside of the manufacturers instructions, what are the other downsides. Owner likes the contractor and wants to keep it :roll: .

 

[dereckbc]

Re: Wye-Wye Transformer

Ron the only thing I can thik of is Triplen and heating. I do not have any experience with wye-to-wye transformers. Can't even think of a reason why to use one.

 

[Guest]

Re: Wye-Wye Transformer

Ron: Is this a single unit type transformer? All three phases on one core? What is the primary voltage?

 

[ron]

Re: Wye-Wye Transformer

This is a single unit type transformer, all three phases on one core. Primary voltage is 480V.
I'm interested in the significance of no neutral on the primary? I'm thinking that will cause some instability in the secondary voltage (gut feeling), but I cant put my finger on the real reason.

 

[Guest]

Re: Wye-Wye Transformer

Both the primary and secondary star points must be connected to the MGN, or supply system neutral.

A neutral is necessary for the primary to stabilize secondary voltage. An unbalanced load, on the primary, with a floating star point will create uneven and high voltages on the primary windings.

 

[Ed MacLaren]

Re: Wye-Wye Transformer

Also, although it may not be applicable in your case, there is the harmonics issue.

Third harmonics readily pass through wye-wye connected transformers.
Delta-wye transformers provide a closed path for third harmonics in the delta winding, thus providing third harmonic isolation between the secondary and the primary.

Ed

 

[Guest]

Re: Wye-Wye Transformer

Would a wye/wye transformer be a separately derived system?

 

[don_resqcapt19]

Re: Wye-Wye Transformer

Bennie,
All transformers.except autotransformers, are SDS. This is because of one simple fact. Current on the secondary side can only return to the secondary side. There is no path for it to return to the primary side. If there is no path to the primary side, there is no common circuit conductor to both the primary and secondary and therefore per the NEC the transformer is SDS.
Don